<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge" />
  <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  <title>Document</title>
</head>

<body>
  <script>
    var cuifan1 = null;
    console.log(typeof 123); // number
    console.log(typeof "123"); // string
    console.log(typeof true); // boolean
    console.log(typeof cuifan); // undefined
    console.log(typeof cuifan1); // object
  </script>
</body>

</html>